The yields of methyl viologen radical cation (MV.+) in the radiolysis of nitrogen-saturated 0.5 mM methyl viologen (MV2+) solutions containing formate were measured using Co-60 gamma-rays, H-1 ions of 2-15 MeV, and He-4 ions of 5-20 MeV. The concentrations of formate were 1, 10, 100, and 1000 mM, which correspond to OH radical scavenging capacities of 3.2 x 10(6) to 3.2 x 10(9) s(-1). Methyl viologen radical cation yields should be stoichiometrically equivalent to the sum of the hydrated electron, H atom, and OH radical yields. This outcome is found in gamma-ray radiolysis except at very high or low formate concentrations. At all formate concentrations, the H-1 and He-4 ion radiolysis, give yields of methyl viologen radical cations that are significantly lower than expected from the results of other aqueous solutions. Possible reasons for the differences are discussed.